Switching converters are used to convert an input voltage to a higher or lower output voltage. Where the input voltage is a rectified line voltage, the input voltage is transmitted by means of a transformer not at a line frequency but at a far higher switching frequency, with which the primary winding is switched to the input voltage. As a result, the size of the transformer can be reduced. A disadvantage of this, however, is high-frequency oscillations, as without appropriate interference suppression such oscillations result in undesired secondary effects.
Often the primary winding is connected to the input voltage by means of two switching elements, to ensure a sufficient dielectric strength. The arrangement is in this case structured symmetrically, so that one switching element is arranged in a positive connection to the transformer and one switching element in a negative connection to the transformer.
Such an arrangement entails additional difficulties, because the respective primary winding halves which are associated with a switching element are not symmetrical. For example, the capacitance between a primary winding half and associated shielding can be determined only by the winding technology used, it being known by experience that deviations of up to 50% can occur. Furthermore, the transformer geometry causes an asymmetry between the primary winding halves, because for example a longer winding circumference of a second primary winding half results in a higher shielding winding capacitance.
In an effort to account for such difficulties, the prior art instructs that the transformer capacitances be recorded precisely and the control of the switching elements adjusted accordingly.